Londoner whose rabbit ate half a duck? (7)
I believe the answer is:
cockney
'londoner' is the definition.
(Cockney is an example)
'rabbit ate half a duck?' is the wordplay.
'rabbit' becomes 'coney' ('coney' can be a synonym of 'rabbit'**).
'ate' is an insertion indicator.
'half a' means to take half the letters.
'duck' cut in half is 'ck'.
'coney' enclosing 'ck' is 'COCKNEY'.
'whose' is the link.
(Other definitions for cockney that I've seen before include "London East-ender" , "Native of London's East End" , "One born within the sound of Bow bells" , "Someone born within earshot of Bow bells" .)